1. Field of the Invention
This invention relates generally to a signal play-back device in which a light beam scans a flat carrier for reading signals recorded thereon, and more particularly to an optical system for such a device by which the scanning light beam keeps a predetermined focus condition on the carrier in spite of the deviation of the carrier from a predetermined position.
2. Description of the Prior Art
It has been proposed to optically record signals, for example, a video signal, on a flat record carrier and/or to optically read signals recorded on the flat record carrier by means of a light beam scanning the surface of the flat record carrier.
In an optical system for such an optical recording and/or reading device, it is required for recording or reading the signals accurately that the light beam which scans the record carrier be always in the proper focus condition, for example, the condition of exact focus on the surface of the record carrier without being influenced by deviations of the carrier from a predetermined position.
However, in the case where a rotatable disc is employed as the flat record carrier and its surface is used for recording signals as usual, if the surface of the rotating disc is inclined in relation to an axis for rotation or if the surface of the rotating disc moves in the direction of the axis of rotation under the influence of the rotation of the disc, which inclination or movement is usually caused to a certain degree in all practical systems, the distance between the surface of the disc and a lens provided for focussing the scanning light beam on the surface of the disc would undesirably vary and, as a result, the scanning light beam would not always keep the proper focus condition on the surface of the disc and the deterioration of the light beam focus would result.
To compensate for the above mentioned deterioration in the focus condition of the scanning light beam, it has been proposed in prior art systems to detect the instantaneous, position of the surface of the rotating disc and, in response to the detected information, to move the lens or the rotating disc to maintain a predetermined distance between the surface of the disc and the lens so as to always keep the scanning light beam in the proper focus condition on the surface of the rotating disc. However, this proposal requires a very complicated and expensive servomechanism, and besides it is difficult for the servomechanism to accurately follow quick variations in the distance because the lens or the disc has a relatively large mass and, therefore, cannot be moved so quickly and accurately. Therefore, the desired compensation is not adequately achieved.
There has been another previous proposal to avoid the deteriorations in the focus condition of the scanning light beam. It is to install the rotatable disc so that its surface is perfectly perpendicular to the axis of rotation and further to absolutely prevent movement of the surface of the rotating disc in the direction of the axis of rotation. This proposal, however, is impossible to realize in any practical system.
Accordingly, the prior art systems have not accomplished sufficient compensation or avoidance of deterioration in the focus condition of the light beam scanning the rotating disc.